1. Field of the Invention
This invention relates to the production of decorated glass-ceramic articles.
2. Brief Description of the Prior Art
Glass-ceramics are produced through the controlled crystallization of glass. A glassforming batch to which a nucleating agent is added is first melted and the melt then cooled to form a glass article of the desired shape. The glass article is then subjected to a crystallizing heat treatment in which the glass is first heated to a temperature in the vicinity of its annealing point to induce nucleation. The nucleated article is then heated to a higher temperature to cause the growth of crystals on the nuclei. The glass-ceramic article is composed of relatively uniformly fine-grain crystals dispersed in a glassy matrix, the crystals comprising the predominant portion of the article. The physical properties exhibited by the glass-ceramic article normally are very different from those exhibited by the original glass; the principal differences being that the glass-ceramics have greater mechanical strength, better thermal shock resistance and lower electrical conductivity, making glass-ceramics useful for dinnerware, cooking surfaces, electrical insulators and other domestic and technical applications. Glass-ceramics have been described in detail in U.S. Pat. No. 2,920,971 to S. D. Stookey.
With the increasing prominance and importance of glass-ceramic materials, it has become desirable and necessary to decorate articles made from such materials. For example, attractive floral decorations on dinnerware makes the articles more appealing to look at and easier to sell. Also, there is a need to decorate glass-ceramic cooking surfaces to mark the location of heaters under the surface.
The art of decorating ceramic articles in an ancient one, with evidence that glazing of lustrous decorations onto ceramics was common in the Persian Empire. A variety of ceramic decorating techniques have been developed during the long history of the art. Colorants in the form of ceramic glazes, particularly colored spinels, have been coated onto glass-ceramic articles, and colorants in the form of stain decorating compositions have also been used. Stain decorating compositions are different from the glazes in that in stain decorating the colorant migrates into the crystal matrix of the glass-ceramic, whereas in glazing the colorant remains on the surface. By far the most popular decorating compositions are the stain decorating compositions. The ceramic glazes have high coefficients of thermal expansion and normally cannot be fired onto low-expansion glass-ceramic bodies without severe crazing and peeling. Stain decorations, on the other hand, in which the colorant diffuses into the body of the glass-ceramic, are far more durable.
Application of stain decorating to glass-ceramic is accomplished by mixing a colorant such as platinum resinate with a flux such as a bismuth compound and adding the mixture to a vehicle such as an oil, alcohol or water, and then applying this composition to the surface of a crystallizable glass article and firing the article to simultaneously crystallize the glass and to fix the color to the surface. Methods for decorating glass-ceramic articles using colorant stains are described in U.S. Pat. No. 3,266,912 to Murphy and U.S. Pat. No. 3,313,644 to Morrissey.
It has been found in stain decorating glass-ceramics that it is extremely difficult to decorate compositions which are free of arsenic and antimony oxides. Arsenic and antimony oxides have been traditionally used in glass compositions as fining agents and act to remove entrapped bubbles of glass which have been generated during the melting of the batch ingredients. However, arsenic and antimony oxides present numerous disadvantages in melting crystallizable glass compositions. Crystallizable glass compositions require rather high melting temperatures on the order of about 200.degree. to 300.degree.F. above the melting temperature of conventional soda-lime-silica glass. To reach these high melting temperatures, it is desirable to supply an electrical source of heat in addition to the conventional form of heating in which oil or gas streams are burned above the glass melt and the heat is conveyed downwardly into the glass mass. The electrical source of heating is direct resistance heating and involves passing an alternating electric current between electrodes usually made of graphite or molybdenum which are immersed in the glass melt. Glass compositions containing arsenic and antimony oxides attack the electrodes making electric melting impractical for such compositions. Removing arsenic and antimony oxides from the compositions, although solving the electrode problem, creates another problem in that crystallizable glass compositions free of the oxides cannot be satisfactorily stain decorated. The resultant stain decoration is fuzzy and very faint.